Electromagnetic radiation at gigahertz and terahertz frequency ranges (e.g., about 100 GHz to about 100 THz) can penetrate many packaging materials from a distance and identify material contained within. For example, terahertz frequencies can facilitate identification of possibly hazardous substances contained within packaging materials. Examples of such packaging materials include shipping containers, storage containers, trucking compartments, etc., that are made of non-conductive materials or sufficiently low conductivity materials.
There are also sizeable economic and social interests in improved security screening methods. Government spending on domestic security alone is estimated at around $75 billion per year. Current screening technologies generally focus on supplying spatial information. For example, the most frequently used security technologies in airports, federal institutions, and other public arenas are x-ray scanners. These technologies can show images of concealed hazards (like knives and guns). However, they provide little to no information about the composition of a potential hazard. Examples of those hazards include explosives, chemical agents, or biological agents. Given that x-rays can be ionizing radiation, there is also the potential for harm to living tissue.
Spectroscopic imaging in the gigahertz and terahertz frequency ranges can be used to identify both the existence of a concealed hazard and its chemical composition. In addition, it is presently believed that electromagnetic radiation in the gigahertz and terahertz frequency ranges does not cause apparent damage to living tissue.
Current terahertz or gigahertz spectroscopic imaging techniques can be time consuming and thus impractical for security screening. Also, there are very few single element or array detectors for these frequency ranges. These include Golay cells, bolometers, and pyroelectric detectors. Each kind of detector has limitations in its ability to be useful both in a wide range of frequencies and as an array. In addition, these kinds of detectors use a thermal response to measure terahertz or gigahertz power. These detectors can be expensive (on the order of $10K to $100K) and slow (response times on the order of millisecond). While photocurrent methods have been employed for detection in the infrared and visible ranges, these photocurrent methods often depend on an above bandgap excitation to create electron-hole pairs which then generate a measureable change in the current or voltage in the device.